1. Field of the Invention
The present invention relates to isomerized hop preparations and to a method for the production of the same.
2. Discussion of Prior Art
For many years dried hop cones have been used in the brewing process to provide beer with a bitter taste and a hoppy aroma. Hop oils which comprise 0.5-2% by weight of the hops are responsible for the aroma, whereas beer bitterness is provided in the main by the .alpha.-acid fraction of hop resins; said .alpha.-acids constituting 4-12% by weight of the hops. Hops are boiled with the brewers' wort during the brewing process and the .alpha.-acids are converted by isomerization into soluble bitter iso-.alpha.-acids.
This process is not particularly efficient and in addition a proportion of the iso-.alpha.-acids are lost during the brewing process such that only 20-40% of the .alpha.-acids are to be found as corresponding iso-.alpha.-acids in the finished beer.
Furthermore hops slowly deteriorate in storage losing both .alpha.-acids and hop oils as a result of oxidation.
The problem of hop storage has been addressed by forming the raw hops into hop extracts by extracting them with a solvent such as hexane, liquid carbon dioxide or methylene chloride and storing the hop extract in sealed containers.
Alternatively, the raw hops are powdered, then pelletised and packaged in evacuated packs. However, in order to utilize such extracts or pellets, they must be boiled in the brewers' wort in order to convert the .alpha.-acids to iso-.alpha.-acids and hence an increase in utilization of only 10% or so (i.e. from 20 to 22%) occurs. Thus, it will be appreciated that the traditional requirement for the conversion of .alpha.-acids to iso-.alpha.-acids requires the .alpha.-acids to be boiled with the wort and hence not only is the conversion to iso-.alpha.-acids low, but further losses ensue as a result of precipitation during wort boiling and cooling, and due to adherence on the surface of the yeast during fermentation.
Hop utilization can be improved by adding isomerized hop extracts to the beer after fermentation thereby avoiding losses of iso-.alpha.-acids during the boiling and fermentation processes. Such extracts are produced by converting the .alpha.-acids present in solvent extracts of hops into iso-.alpha.-acids by boiling with dilute aqueous alkali salts such as sodium or potassium hydroxides or carbonates for 30-60 minutes. However, this has the con-commitant difficulty that hop oils tend to be degraded during boiling and it is normal practice to remove the oils during the preparation of isomerized extracts.
Alternatively, solvent extracts of the hops may be treated with magnesium or calcium salts to precipitate the .alpha.-acids from suitable solvent extracts thereof. The salts are then heated to effect the isomerization reaction and to produce the corresponding iso-.alpha.-acids as their calcium or magnesium salts. These salts have only limited solubility and require an extended contact with the beer. Of course isomerized extracts can be added to the boiling wort and this indeed results in a marked improvement in hop utilization. However, the production of these extracts involves a multi-stage process of solvent extraction purification, preparation of a fraction rich in .alpha.-acids, isomerization and hop oil removal. Hence the processing costs tend to be high and it is not commercially attractive to use isomerized extracts during wort boiling.
To this end hop pellets have been prepared as discussed in U.S. Pat. No. 4,123,561. In this Disclosure the alkaline earth oxides, magnesium and calcium oxide, are admixed with powdered hops and pelletised. In this process .alpha.-acids are converted to their more stable salts, and as a result heat is generated and some isomerization occurs. Subsequent anaerobic heating of the pellets in a controlled manner results in the production of isomerized pellets in which over 90% of the initial .alpha.-acids are converted into the corresponding magnesium or calcium salts of the iso-.alpha.-acids. Whereas such a process is suitable for the production of iso-.alpha.-acids, hop oils tend to be degraded during heating thereby leading to a cheesy aroma. This tends to lead to problems in brewing and hence the process of U.S. Pat. No. 4,123,561 is of limited applicability. Furthermore, a pelletisation step is required in this process. This gives pellets which tend to remain coherent in the brewers' wort and hence a significant percentage of the available iso-.alpha.-acids do not in fact get utilized in the brewing process.
Extrusion cooking is a widely used process in the food industry for the production of breakfast cereals, pet foods and various snack products. An extrusion cooker in its simplest form comprises an Archimedian screw rotating inside a closed cylindrical barrel which can be heated on a zone basis to control temperatures along the length thereof. Generally the product in its precooked form is moistened and introduced into the cooker where it is cooked by a combination of pressure, heat and mechanical shear, said shear including being forced through a die at between 3.5 and 25 Kg/cm.sup.2 at the end of the cooking process. Forcing the product through the die causes an immediate expansion of the product on exit due to the flashing of steam under pressure.